Interlocking joints in rings, cylinders, and other tubular structures are customarily assembled during or after the primary fabrication of the cylinder itself. Tangential forces tending toward in-plane separation of the joint are resisted by the interlocking geometry of the joint structure itself or by friction in simple tongue and groove structures. That is to say, this structure resists the direct pulling apart of the joint edges, and of course also any lateral in-plane movement. In addition, friction between the interlocked edges of the joint, along with the stiffness of the total structure, provides resistance against radial or out-of-plane displacement of the interlocked joint ends, even though there is no basic joint geometry which prohibits the out-of-plane dislodgement of the joint. This is to say, these frictional forces tend to discourage one seam edge from "popping out" relative to the other.
In thin wall structures, particularly those having a large diameter-to-thickness ratio, the joint friction and the structural stiffness are sometimes insufficient effectively to resist out-of-plane dislodgements of the joint when the structure encounters impact or crushing loads during transit or use. Also these out-of-plane joint dislodgement problems can further be aggravated by cut edges of the joint which are off-square, due to normal sheared edge geometries or due to breakdown of the edge corner during the engagement of the interlocking joint. These undesirable conditions further reduce the holding friction, and sometimes even provide a preferred direction of radial displacement ("dislodgement") under shock or crushing loads.
It is an object of this invention to provide means for strengthening the joint, particularly against radial or out-of-plane joint rupture, whereby a joint can be manufactured in an expedient and inexpensive rolling and stamping operation and with positive restraints in all three axes of movement relative to the plane in which the seam is formed.